1. Field of the Invention
This invention relates to a method for destroying cyst of noxious plankton.
2. Prior Art
A ship having no or little load is difficult to maintain its equilibrium because of its lowered waterline. Therefore, such a ship is usually loaded with ballast water to maintain its equilibrium thereby assuring safe navigation thereof in the ocean. The ballast water is unloaded before the ship is loaded with cargo in a port of destination or before it arrives therein.
The ballast water is seawater pumped up and accommodated within a watertight compartment defined inside the ship for the above reason. The ballast water may include noxious plankton depending on the sea area from which it is pumped up, and may cause shellfish poison or red tide when discharged from the ship in the offing or harbor.
It is well known that a red tide due to an explosive multiplication of such noxious plankton pollutes the sea, causing fish and shellfish to die. This seriously damages the cultured fishery in particular. It is to use hydrogen peroxide, calcium peroxide or a peroxyhydrate as a controlling agent for red-tide plankton such as Chattonella marina. Gymnodinium nagasakiense, and the like [refer to Japanese Unexamined Patent Publication No. 141142/1980; "Screening of removal agents of a red tide plankton Chattonella marina--with special reference to the ability of free radicals derived from the hydrogen peroxide and polyunsaturated fatty acid", Nippon Suisan Gakkaishi 55 (6) 1075-1082 (1989); "Toxic effects of hydrogen peroxide on Gymnodinium nagasakiense and fishes", Bull, Fac. Bioreso, Mic Univ. No. 4 165-173 (1990)].
Various kinds of noxious plankton belonging to the following orders of classes are known.
1. Cyanophyceae PA0 2. Cryptophyceae PA0 3. Dinophyceae PA0 4. Bacillariophyceae PA0 5. Raphidophyceae PA0 6. Chrysophyceae PA0 7. Haptophyceae PA0 8. Euglenophyceae PA0 9. Prasinophyceae PA0 10. Chlorophyceae
(i) Chroococcales PA1 (ii) Nostocales PA1 (i) Cryptomonadales PA1 (i) Prorocentrales PA1 (ii) Dinophysiales PA1 (iii) Gymnodiniales PA1 (iv) Noctilucales PA1 (v) Peridiniales PA1 (i) Centrales PA1 (ii) Pennales PA1 (i) Raphidomonadales PA1 (i) Ochromonodales PA1 (ii) Pedinellales PA1 (iii) Dictyochales PA1 (i) Isochrysidales PA1 (ii) Prymnesiales PA1 (i) Eutreptiales PA1 (ii) Euglenales PA1 (i) Nephroselmidales PA1 (ii) Pterospermatales PA1 (iii) Pyramimonadales PA1 (i) Volvocales
(i-i) Coscinodiscineae PA2 (i-ii) Rhizosoleniineae PA2 (i-iii) Biddulphiineae PA2 (ii-i) Araphidineae PA2 (ii-ii) Rhaphidineae
The noxious plankton belonging to the above orders is classified into two types depending on its reproduction manner: asexual reproduction by binary fission, or sexual reproduction only between different mating types (+,-) to form cyst. The cyst in the latter corresponds to a seed of plant and germinates under certain conditions to become plankton. The exine of cyst is completely different from a cell wall of plankton in that it is extremely solid. Accordingly cyst can be in a dormancy for several years or more without dying even under severe conditions such as dark environment, oxygen-free environment and the like, under which plankton cannot be alive. Thus, the cyst is completely different from plankton in physiology, ecology and even morphology.
The phenomenon such that shellfish becomes poisonous due to shellfish-poisoning plankton has become tangible since 1987 or so in the sea off Japan. Recently, it was confirmed that cyst of shellfish-poisoning plankton was inhabiting the ballast water discharged from a Japanese oceangoing ship. Such cyst is presumed to be a cause of the occurrence of poisonous shellfish, and this problem is given consideration by other countries. Such a phenomenon has a tendency to be extended and prolonged, and hence it is desired to promptly take measures therefor and especially to develop an effective technical means.
The present inventors have investigated the facts so far known concerning the substance of the problem caused by the cyst of shellfish-poisoning plankton present in the ballast water. Although it is proved that shellfish is poisoned by the above-noted shellfish-poisoning plankton, there are things left unknown in the correlation between cyst and plankton. For example, the conditions, rate and time of germination of cyst are inconstant depending on the kind of cyst, and the dormancy period thereof is not precisely known yet. Further, tests revealed that cyst could not be destroyed by a plankton-destroying agent such as copper sulfate, montmorillonite or the like at a concentration for destroying plankton, by a ultrasonic treatment for a predetermined period of time for destroying the same, or by hydrogen peroxide under the conditions therefor.